Conventional gas lasers include a sealed plasma tube for confining a gaseous lasing medium. In a first class of conventional gas lasers, both ends of the plasma tube are sealed with Brewster windows (transmissive elements oriented with respect to the laser beam at the Brewster angle). Throughout the specification, including in the claims, the term "window" will be used in a broad sense to denote any transmissive element which is impervious to the flow of gas.
In the mentioned first class of gas laser systems, the end mirrors defining the ends of the optical cavity are mounted outside the plasma tube, at locations spaced from the Brewster windows. Thus, the optical cavity is not completely sealed, and intra-cavity beam control elements such as wavelength selecting prisms, single frequency etalons, acousto-optic or electro-optic modulators, nonlinear crystals for second harmonic generation, mode control devices (such as aperture wheels having several different hole sizes), polarizing elements, and the like, may be conveniently positioned in the non-sealed portion of the optical cavity between the Brewster windows and the end mirrors.
However, conventional lasers in this first class have two important disadvantages. The materials comprising the Brewster windows are directly exposed to the harsh plasma tube environment, and particularly, to hard UV radiation produced by electrical discharge in the extremely low pressure plasma tube atmosphere. As the windows absorb hard UV radiation over time, their optical characteristics degrade in two principal respects. First, their transmissivity decreases with time. Second, the distortions they induce into the transmitted laser beam become increasingly severe with time.
A protective coating could be applied to the window to shield the window from hard UV radiation. However, such a protective coating would need to be extremely thin (typically less than about 100 Angstroms) in order to keep to an acceptably low level the loss the coating introduces into the beam. Such a thin coating does not provide a long lasting protection to the window. On the other hand, it is impractical to coat a sealed Brewster window with a sufficiently thick coating to protect it from the damaging UV radiation.
For example, due to the mentioned problems, conventional ion laser plasma tubes of the described type (tubes sealed at each end with Brewster windows) have short lifetimes, typically in the range from about 1000 hours to about 1200 hours when operated at full power in the ultraviolet wavelength region.
It will be appreciated that another significant disadvantage of the first class of conventional plasma tubes (sealed at both ends with Brewster windows) is that the Brewster windows introduce unavoidable losses into the laser system, since they inevitably transmit less than 100 percent of the radiation incident thereon (due to manufacturing flaws and imperfect alignment, among other reasons).
In a second class of conventional gas laser systems, a substantially totally reflective mirror is attached directly to one end of the plasma tube and a partially reflective mirror is attached directly to the other end of the plasma tube, to define a sealed laser optical cavity. This approach eliminates the problems discussed above that are associated with sealed windows. However, this class of gas laser systems is subject to the important disadvantage that, because the optical cavity is completely and permanently sealed, intra-cavity beam control elements cannot conveniently be positioned within and removed from the optical cavity.
Previously, it had not been known how to solve the above-mentioned problems of Brewster window degradation (and corresponding short plasma tube lifetime) and optical losses due to use of Brewster windows, while at the same time permitting convenient insertion and replacement of intra-cavity beam control elements.